Preparation of purified brine



Patented Dec. 30, 1947 PREPARATION OF PURIFIED BRINE Rock L. Comstock, Weeks, La., assignor to Bay Chemical Company, Inc., New Orleans, La., a

corporation of Louisiana No Drawing. Application January 25, 1944, Serial No. 519,651

Claims.

This invention relates to the preparation of purified brine and more particularly to the preparation of sodium chloride brine low in calcium and sulfate radicals from crude sodium chloride or rock salt containing substantial quantities of calcium sulfate.

For certain purposes, such as for regeneration of zeolites for water softeners, the production of chlorine by electrolysis, and in many dye processes, it is extremely desirable if not necessary to reduce the concentration of calcium radical and also the sulfate radical in brine to extremely low values. In conventional processes of making such pure brines it has been the practice to form a brine by dissolving crude salt in water and then add certain reagents to the brine, in order to precipitate the undesired compounds. For example, calcium has been precipitated as calcium carbonate by adding an alkali metal carbonate such as sodium carbonate, magnesium has been preci itated by adding alkali metal hydroxides, and the sulfate radical has been precipitated as barium sulfate by adding a soluble barium salt. These operations result in the formation of bulky precipitates which must be filtered or settled out of the resulting brine. The preci itation of the sulfate radical is particularly difiicult in that it usually involves a separate and expensive step and the presence of the sulfate radical is frequently tolerated rather than its removal undertaken.

In accordance with the present invention it has been found that brines low in sulfate radical as well as calcium radical may be prepared by employin as inhibiting agents smallamounts of soluble compounds such as alkali metal carbonates or phosphates which form insoluble compounds with ca cium radical. That is to say, the agents employed will react with calcium sulfate in aqueous solution to precipitate a water-insoluble calrequired to produce a brine low in calcium radical reduced over that of prior processes but the same agent reduces the amount of sulfate radical,

contrary to what would be expected from said prior processes.

It is, therefore, an object of the present invention to provide an improved process of obtaining brines low in calcium compounds from crude sodium chloride containing substantial amounts of such calcium compounds. 4

An object of this invention is to provide a method of inhibiting the solution ofcalcium sulfate impurity in salt by dissolving the salt in the presence of precipitant of calcium radical, the dissolving preferably being conducted at a pH of 7 or more.

Another object of the invention is to provide an improved method of producing sodium chloride brine low in sulfate content from crude sodium chloride having a substantial calcium sulfate content.

Another object of the invention is to provide an improved process of obtaining a purified sodium chloride brine from crude sodium chloride in which soluble agents which form insoluble calcium compounds with calcium sulfate are employed to inhibit solution of calcium sulfate contained in the crude sodium chloride.

Another object of the invention is to provide a process of making a purified brine in which the quantity of agents required to produce a brine substantially free of calcium and sulfate radicals is markedly reduced.

A further object of the invention is to provide an improved process of obtaining sodium chloride brine substantially free of sulfate radical and calcium radical from crude sodium chloride having a substantial calcium sulfate content in which the solution of the sodium chloride is carried out in the presence of alkali metal compounds forming insoluble compounds with calcium sulfate.

A still further object of the invention is to provide an improved solid salt product containing a solution inhibiting agent for calcium sulfate contained in said salt.

In carrying out the process of the present invention, the brine is formed in the first instance by dissolving a salt in the presence of the agents which react with calcium radical to produce insoluble compounds. This is preferably accomplished by dissolving sodium chloride from crude salt or rock salt in the presence of such agents, although the invention is applicable to the preparation of brines from salts other than sodium chloride. It has been found, for example, that the sulfate content of brines produced by dissolving such salt containing substantial amounts of calcium sulfate, in the presence of small amounts of sodium carbonate or trisodium phosphate results in a much lower sulfate content than that obtained if the same amount of sodium carbonate or phosphate is added to the brine after formation thereof and the resulting insoluble precipitates removed from the brine. Adding the agents of the present invention to the brine after formation thereof has little effect upon reduction of the total amount of sulfate radical, whereas operations in accordance with the present invention definitely inhibit solution of sulfate radical.

The soluble agent employed in the present invention is preferably dissolved in the water prior to dissolving the rock salt or other impure sodium chloride containing sulfate and calcium radicals, but may be added to and mixed with the impure sodium chloride prior to the dissolving operation. Best results are, however, obtained by first dissolving the agent in the water used to dissolve the impure sodium chloride.

It has also been found that the pH of the water 3 during the dissolving operation is of importance. The content of calcium compounds including calcium sulfate, in the brine increases rapidly as the pH of the water drops below 7. Thus, the

form application of the agent to the crude salt. However, a small amount of powdered agent or a concentrated solution of the agent may be thoroughly admixed with crushed rock salt or other addition of an alkaline material such as sodium' 6 'comminuted salt prior to contacting it with wahydroxide to water having a low pH value to ter for dissolving the salt. In any event the disbring its pH up to at least '7 prior to dissolving solving operation is carried out in the presence of the crude salt materially reduces the content of the agent. calcium radical and sulfate radical in the resultr The following table shows typical results illusing brine but further additions of sodium hydrox- 10 trating the difference obtained (1) Without an ide have very little further effect. For example, inhibiting agent, (2) adding an inhibiting agent the amount of calcium sulfate present in the to the brine after dissolving the salt, and (3), (4) brine when employing a distilled water having a dissolving the salt in the presence of an inhibiting pH of 5.2 is approximately three times that when agent, the particular agent selected being sodium a water having a pH of 7 is employed. The pI-l of carbonate.

Table I Agent Resulting Brine Parts Residual CaSOi Type 3 5?; ii gg When added iiiiiit t i gg parts water dissolved ig (may rsalt dissolved dlssolved 1. None 0 0 0.167 0.069 2. Sodium Carbonate 0.0028 0.1 To brine after dissolving salt 0 13S 17 0.024 3. Sodium Carbonate 0.0028 0.1 To salt before adding water 0. 042 75 0.011 4. Sodium Carbonate 0.0028 0.1 To water before adding salt 0.036 78 0.012

Water can be raised with alkaline compounds In securing the data for the above table the imother than hydroxides or a natural water havpure salt containing calcium sulfate was added ing a pH of '7 or above can be employed to give to Water in an amount equal to one part salt to a similar reduction in calcium sulfate in the final N three parts water and the mixture agitated for brine. The employment of alkaline compounds 9 5 minutes so as to dissolve substantially all the such as basic alkali metal carbonates or phossalt and the resulting brine then filtered. Any phates which react with calcium radical to form further p ipi for i after fi t ing w s insoluble compounds is, however, preferred for tled and removed. The sulfate content of the increasing the pH of a Water which has a pH filtered brine is reported as calcium sulfate and below 7 as h compounds also have the add}- the calcium content as calcium oxide. It will tional effect discussed above of further decreasing be note'd a addition of h agent '1 the brine the amounts of calcium radical and calcium sulafter dlssolvmg the as run 2 dld not have fate in the resulting brine. Addition of these mam on reductmn of ,sulfate content but terials to a water which already has a pH above 7 did mater ally reduce the calcium content. Howalso produces the additional effect above referred ever adqmg the agept t.e1ther Salt or the to. For example, by employing an alkali metal water pnor to the dissolving operatlon not l carbonate or alkali metal phosphate the amount gl reduced dilhlel callc Content of sulfate radical in the final brine can be reduced marke y reduce t e Su fate 1 Ion toapproximately one-fourth or one-fifth that proof the agent to the Water gave Somewhat better d 7 results than adding the agent to the salt as shown uced when hydroxides alone are employ ed. If the particular carbonates or phosphates or other by companscn of runs 3 and g The following table illustrates typical results compounds selected do not tncrease the pr: to obtained on the same impure salt without em or greater, a strong alkah such as an alkali ploymg an inhibiting agent and by employing metal h can be employed m comunctlon the same amounts of sodium carbonate and tritherewlth {ncrease the sodium phosphate. Anhydrous sodium carbonate The preclplmmg agent may be added to either (NazCOs) and hydrated trisodium phosphate the crude salt or to the water employed to dis- (Na3PO4 12H2O) were used throughout eXPerisolve the crude salt. The preferred method is to ments and 1 Weight and ratios refer t these a d t e a ent 11 the Water p Oyed for the disforms. Other forms containing equivalent solving operation as thi results in a more uniamounts of inhibiting agent may be used.

Table II Agent Resulting Brine Parts Caso4 Residual Type 52? ioiifiai When added ififitt l l i 132x252 gallon of of NaCl parts ciency parts water dissolved Nam of salt dissolved dlssolved o 0 .285 0.122 Sodium Carbonate .00556 .200 To water before adding salt .055 81 0.0021 Trisodium Phosphate .00556 .200 do .030 88 0.0020

It will be noted that both of these agents decreased the calcium content by about the same amount while the trisodium phosphate was somewhat more efiective as to inhibiting solution of ulfate radical, the percent efiioiency of the trisodium phosphate being 88 while that of the sodium carbonate being 81. In general, the tri sodium phosphate is more effective for this purposeasshown by the followingtable which also illustrates the effect on sulfate'content of Vary.- ing the-am unt of a ent- 6 compounds which might be present in natural water. However, natural waters, even hard waters. can be employed to produce substantially the same results, particularly if enough precipitating agent isemployed to provide the excess above mentioned over the total amount of calcium r dical in both the water and dissolved from the salt. .Alsmless alkaline agents, such as sodium bicarbonate or disodium phosphate, can be employed to give substantially similar results if a strong alkali such as an alkali metal hydroxide is Table III Agent Resulting Brine er s a 4 gfi per 100 dissolved Per Cent Type gallon ffgf when added z g Efficiency dissolv d aOl dissolved 00092 033 To water before adding salt 066 77 82a is: 82% a g o sdmm cart-1mm .0037o .133 do .041 86 00463 107 d 052 82 005 56 .200 ..d V--..---, .056 77 00092 033 J 064 78 88%? it, a: s; Triscldmm '00370 I133 1111110-. I032 89 00463 167 d 030 90 00556 200 --..--d0. .027 91 The salt used in the tests shown in Tables I, II and III was of mine run grade, having 1.31% calcium sulfate and the following screen analysis: on .2 mesh-21.7%; on 4 mesh-54.2%; on 9 mesh-18.9%; on mesh3,2%; thru 20 mesh- 2.0%.

When thedata of the above table are plotted as a curve it is found that with sodium carbonate the. amount of sulfate reaches. a minimum at .approximately 0.12% sodium carbonate .based on the amount of salt dissolved or 0.00344 pound per gallon of Water employed and that the amount of sulfate decreasesto a nearly constant value at approximately the same figure when employing trisodium phosphate. These amounts will vary somewhat with different salts being dissolved and difierent agents but good results will usually be obtained between approximately 0.0014 to 0.0057 lb. of inhibiting agent per gallon of water em-- ployed or between approximately 0.051and 0.21% of agent based on .the amount of salt dissolved. when, as is usually the case, approximately 1 part of salt by weight is dissolved in 3 parts of water. Best results are obtained by employing between app ximately 0. 0 8 t 0.0034 1b. of a ent per all nof water or 0.10. to 0.12% of agent based on the amount of salt dissolved. For trisodium phosphate the smaller amounts givenin the preceding sentence are preferred while the larger amounts are preferred for sodium carbonate. "It is found that a slight excess of the agent should be present in the resultant brine for best results which test is applicable to operations on impure salts having widely varying sulfate and alkaline earth metal cont nt al o gh the r n es above gi en re nerally applicable. Thus, an excess of agent residual in the brine ranging between approximately 0.001 to 0.0017 pound pergallon of water over' that consumed in precipitating calcium radical dissolved from the salt gives best results. In general, the.amoun t of agent necess ry to pr d ce such an excess will n t xceed 0. of the salt dissolv In Tables II and distilled water was used to eliminate any effect of alkali metal or calcium added in sufiicient amounts to bring the pH of the water to 7 or above. Also, mixtures of various agents either of the same alkalinity or of difierent alkalinity can be utilized so long as the resulting pH is at least 7.

To illustrate the results of dissolving salts of dilferent degrees of division and having different sulfate contents, the following tables are given.

Table IV Parts CaSO dissolved per Per cent gg g salt Per cent Type of Salt CaSO v Efliciency in salt Water Water alone plus agent Table V Per Per Per Per Per Per Per Cent Cent Cent Cent Cent Gent Cent ggg On On On On On On On g 4 9 20 40 60 80 100 Screen O 0 0 0.9 6. 8 11.6 26. 5 54. 2-100 0 0 .0 47. 2 41. 6 4. 4 6. 7-80 Preferred" 0 0 1.9 89. 8 3. 5 3 6-80 Gap-"u, 0 0 91. 2 4. 5 1. 2 5 2. 6-80 #1 81. 2 1'8. 5 3-9 Table IV shows theamount of calcium sulfate dissolved when an inhibiting agent was employed as compared to the amount when water alone was employed whereas Table V shows the particle sizes of the salt dissolved. The inhibiting agent in all of the examples of Table IV was .00556 pound of trisodium phosphate per gallon of Water or .2% based on the amount of salt dissolved. It will be noted that the per cent efiiciency of inhibiting the solution of sulfate is apparently affected more by the size of the salt particles than by the amount of calcium sulfate in the salt. Thus, the salts having the greatest amount of fines gave the poorest per cent efficiency and this indicates that the inhibiting action may be a surface coating phenomenon since the effect appears to depend upon the surface area of the particle. The same thing can be shown by taking two samples of a rather coarse salt, dissolving one without grinding and grinding the other before dissolving. Thus, using the same agent, namely .0027? lb. of trisodium phosphate per gallon of water or 0.1% of the salt dissolved, a coarse sample of salt having particle sizes such that 81.2% was retained on a No. 4 screen, 18.5% on a No. 9 screen, with 0.3% going through, gave a sulfate solution inhibiting efficiency of 88.5%, whereas a ground sample of the same salt having particle sizes such that none was retained on a No. 26 screen, 35.8% on a No. 40 screen, and 42% on a No. 100 screen, with 22.2% going through, gave a sulfate solution inhibiting efficiency of only 78%. These figures indicate that better results are obtained with coarser salts.

It is apparently the presence of the phosphate, carbonate or similar radical during dissolving of the sodium chloride from the crude salt in conjunction with the pH of the water employed being above 7, which inhibits the dissolving of calcium sulfate and other calcium compounds. It will, therefore, be further apparent that other carbonates such as bicarbonate or other phosphates such as disodium or monosodium phosphates can be employed and if necessary alkali metal hydroxides or other strong alkali can be employed in conjunction therewith to insure that the pH of the water during extraction of the crude salt is at least as high as 7 and preferably in the neighborhood of 8 or greater. As stated above, a major reduction of calcium sulfate as well as other calcium compounds in the resulting brine can be obtained by increasing the pH of waters having a pH below '7 by the addition of substantially any alkaline material. However, increasing the pH above 7 with such an alkali as sodium hydroxide has very little effect whereas the addition of alkali metal compounds which form insoluble compounds with calcium compounds provides a further decrease of calcium sulfate and other calcium compounds in the resulting brine. Thus, the alkali metal carbonates and phosphates are particularly useful, the latter giving better results than sodium carbonate.

The step of dissolving the sodium chloride containing calcium sulfate so as to inhibit the dissolving of the calcium sulfate as well as any other calcium compounds which may be present may be carried out in substantially any desired manner so long as the inhibiting agent is present during the dissolving operation. Thus water containing the inhibiting agent may be added to a body of salt or conversely the crude salt can be added to a body of water and the dissolving operation may be carried out with or without agitation. Alternatively the inhibiting agent can be added to the impure salt either in powder form or as a concentrated solution and the dissolving operation carried out as above described. The impure solid salt containing the inhibiting agent can be made and sold as a product of commerce so that purchasers may make the purified brine of the present invention by merely adding water and then filtering or settling.

While I have disclosed the preferred embodiments of my invention it is understood that the details thereof may be varied within the scope of the following claims:

I claim:

1. In a method of preparing brine in which the materials brought into contact to form said brine are water and a solid water-soluble salt containing a substantial amount of watersoluble calcium sulfate as an impurity, the improvement which comprises forming a brine low in sulfate radical and calcium radical by adding to one of said materials before bringing said ma... terials into contact a small amount of watersoluble alkali metal compound selected from the group consisting of alkali metal carbonate and alkali metal phosphate and which reacts with calcium sulfate in aqueous solution to precipitate a water-insoluble calcium compound, thereafter bringing said materials into contact to dissolve salt in said water and separating residual solid material from the resulting brine.

2. The method defined in claim 1, in which the amount of water-soluble alkali metal compound added to one of said materials is between approximately 0.051 and 0.25% by weight based on the salt dissolved in said water.

3. The method defined in claim 1, in which the amount of water-soluble alkali metal compound added to one of said materials is between approximately 0.0014 and 0.0057 pound per gallon of water used to dissolve said salt.

4. The method defined in claim 1, in which the water used to dissolve said salt has a pH of at least '7.

5. In a method of preparing brine in which the materials brought into contact to form said brine are water and solid sodium chloride containfng a substantial amount of water-soluble calcium sulfate as an impurity, the improvement which comprises forming a brine low in sulfate radical and calcium radical by adding to one of said materials before bringing said materials into contact a small amount of water-soluble alkali metal compound selected from the group consisting of alkali metal carbonate and alkali metal phosphate and which reacts with calcium sulfate in aqueous solution to precipitate a waterinsoluble calcium compound, thereafter bringing said materials into contact to dissolve sodium chloride in said water and separating residual solid material from the resulting brine.

6. In a method of preparing brine in which the materials brought into contact to form said brine are water and solid sodium chloride containing a substantial amount of water-soluble calcium sulfate as an impurity, the improvement which comprises forming a brine low in sulfate radical and calcium radical by adding to one of said materials before bringing said materials into contact an amount between approximately 0.051 and 0.25% by weight based on the sodium chloride dissolved in said water of water-soluble alkali metal salt selected from the group consisting of alkali metal carbonate and alkali metal phosphate and which reacts with calcium sulfate in aqueous solution to precipitate a water-insoluble calcium compound, thereafter bringing said materials into contact to dissolve sodium chloride in said water and separating residual solid material from the resulting brine.

7. The method defined in claim 6, in which the water-soluble alkali metal salt added to one of said materials is an alkali metal carbonate.

8. The method defined in claim 6, in which the water-soluble alkali metal salt added to one of said materials is sodium carbonate.

9. The method defined in claim 6, in which the Water-soluble alkali metal salt added to one of said materials is trisodium phosphate.

10. The method defined in claim 6, in which the water used to dissolve said solid sodium chlo- 5 ride has a pH of at least 7.

ROCK L. COMSTOCK.

REFERENCES CITED The following references are of record in the 10 file of this patent:

UNITED STATES PATENTS Number Name Date Re. 1,290 Spencer Mar. 18, 1862 15 Number Number 

